This application is based on French Patent Application No. 00 16 938 filed Dec. 22, 2000, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The invention relates to an optical architecture for a wide-angle observation telescope, in particular a telescope intended to be installed on board a vehicle. That vehicle is a space satellite, for example, using the telescope to observe terrestrial areas, as in the SPOT observation program, for example.
2. Description of the Prior Art
As is known to the person skilled in the art, current requirements in terms of observation from a satellite imply a spatial observation resolution over an area of a few meters within a field of the order of 40 to 60 kilometers, for example. There is also a requirement for the telescope to have no central obscuration, such as exists with some prior art telescope architectures.
These requirements in terms of observation and absence of obscuration can be met by three-mirror anastigmats (TMA). FIGS. 1 and 2 show an appropriate prior art architecture for TMA telescopes. The telescope includes an off-axis, aspherical and concave first mirror 1 which therefore has no symmetry of revolution and reflects the beam toward a second mirror 2 so that it converges slightly, as can be seen in FIGS. 1 and 2. The mirror 2 is an aspherical, convex and off-axis mirror, unless it is located at the pupil of the telescope, in which case it is obtained from a deformed spherical shape. The beam that it receives is reflected in the form of a divergent beam toward an aspherical, concave and off-axis third mirror 3, as also shown in the two figures referred to above. The mirror 3 receives the divergent beam reflected by the mirror 2 and focuses it in an off-axis focal plane PF. The system formed of the three mirrors 1, 2 and 3 constitutes an anastigmatic optical imaging system in which the image field is off-axis. This kind of system is bulky with dimensions L1, L2 and L3 (see FIGS. 1 and 2) respectively equal to 550 mm, 900 mm and 400 mm, for example, for a telescope with a 160 mm pupil and a focal length of 610 mm.
This kind of TMA telescope has a number of disadvantages. It implies the production of at least two concave, aspherical and off-axis mirrors, and these are costly because they have an elongate shape and are difficult and time-consuming to produce. The assembly obtained is heavy, which is an undoubted disadvantage in the case of equipment intended to be used on board a space satellite. Also, the optical architecture defined for the telescope is sensitive to eccentricity.
The invention therefore proposes an optical architecture for observation telescopes, in particular for telescopes intended to be installed on board a vehicle, such as a space satellite, for observing terrestrial areas, which architecture includes:
a concave and off-axis mirror which is aspherical or possibly spherical and reflects in the form of a convergent beam a beam consisting of radiation that it receives from a terrestrial area that it is observing,
a dioptric and achromatic aperture correction plate inserted on the path of the convergent beam reflected by the mirror,
a dioptric and achromatic field correction plate inserted on the path of the convergent beam reflected by the mirror on the downstream side of the aperture correction plate relative to the mirror, and
a pupil on the path of the reflected convergent beam to obtain an off-axis field of view preventing central obscuration.
In a variant of the invention, the optical architecture of the telescope includes:
a concave and off-axis mirror which is aspherical or possibly spherical and produces two images with a particular angular separation and which reflects as a convergent beam each of the two beams consisting of radiation that it receives from a terrestrial area that it is observing,
a dioptric and achromatic aperture correction plate inserted on the path of the convergent beams reflected by the mirror,
a dioptric and achromatic field correction plate inserted on the path of the convergent beams reflected by the mirror on the downstream side of the aperture correction plate relative to the mirror, and
a pupil on the path of the reflected convergent beams to obtain two fields of preventing central obscuration, the pupil being adapted to accommodate simultaneously two sighting directions each corresponding to one of the reflected beams.
In one embodiment common to both architectures according to the invention the pupil is on a front face of the aperture correction plate which receives the convergent beam or beams reflected by the mirror.
The aperture correction plate and the field correction plate are achromatic and without power or virtually without power.
The aperture correction plate and the field correction plate are each respectively made up of lenses of at least two different types of glass.
The indices of the glasses of at least one of the correction plates can be chosen to take into account variations caused by temperature.
One embodiment of the invention includes a focussing mechanism operating on the lenses of the field correction plate.
One embodiment of the invention includes an auxiliary operating on the beam, such as a beam splitter, with dichroic prisms and divolis (optical line dividers), enabling the beam to be split spectrally, an image derotator, or a splitter plate for dividing the beam to obtain images in different focal planes.
The invention, its features and its advantages are explained in the following description, which is given with reference to the accompanying drawings.